Modern humans originated in Africa approximately 150,000-200,000 years ago. Fossil remains prove that they had moved north of what is now the Sahara Desert sometime before roughly 90,000-120,000 years ago.
How did they get across the desert? A team of scientists based in the United Kingdom and Libya, led by Anne Osborne at the University of Bristol, provide evidence for a past freshwater corridor out of Africa.
The migration controversy.
It has long been proposed that modern humans migrated out of Africa along the Nile River, roughly 130,000 years ago. However, fossil remains prove that humans were already widely dispersed across the north African coast at this time.
In combination with other archaeological evidence, and the question of whether or not the Nile offered a hospitable corridor during this period of time, it is reasonable to question whether other, and perhaps more important, migration routes out of Africa were used instead.
Past climate poses a possibility.
If the Nile was not the primary corridor, it is not immediately obvious what was used instead. The past climate of the Sahara offers insight into this question.
The Sahara was not always so arid; at times, it was much more humid. In fact, evidence exists for past civilizations, roughly 10,000-5,000 years ago, in what is now the middle of the desert.
One period of time in which the Sahara was wet was during the most recent interglacial period. This encompasses the time of the first modern human migration out of Africa.
A consistent, uninterrupted, freshwater corridor across the Sahara, other than the Nile, would provide a clear alternate route out of Africa. Anne Osborne and coworkers provide evidence for such a corridor in the past.
Molecular isotope data.
Molecular isotope data, from oxygen and neodymium, provides evidence of a large influx of freshwater throughout the eastern Mediterranean Sea during the time of interest.
An anomalous ratio of oxygen-18 to oxygen-16 present approximately 124,000-119,000 years ago suggests that much more freshwater was coming into the Mediterranean Sea at that time. This anomaly is particularly evident around the western Ionian Sea (near Italy), suggesting a source near Libya.
However, while tantalizaing, this alone is not conclusive evidence for a past river out of Africa. Neodymium isotopes provide further evidence.
Modern water flowing into the eastern Mediterranean through the Strait of Silicy, recorded in plankton fossils, contains a low ratio of neodymium-143 to neodymium-144. This means that the silt currently being dumped into the Mediterranean is largely from the old continental crust depositions that surround the North Atlantic Ocean.
However, a higher neodymium isotope ratio is observed in the same time period as the oxygen isotope ratio anomaly. Dust from both the Sahara and the Nile possess much lower neodymium isotope ratios, suggesting that they are not the source of the past silt. This suggests a local transient source of silt back then.
Satellite data.
The molecular isotope data suggests that, over the period of time in which modern humans migrated out of Africa, silt was being dumped into the Mediterranean from somewhere other than either the Atlantic or the Nile. Satellite images show where it came from.
These satellite images show buried river channels leading into the Gulf of Sidra (Libya). They are part of a system that was more than 800 km long, of channels sometimes more than 5 km wide, from the Tibesti Mountains to the Mediterranean.
Crucially, neodymium isotope ratios in the Tibesti Mountains are consistent with the ratios observed in the Mediterranean roughly 124,000-119,000 years ago. During this time, freshwater came from neither Lake Chad (probably drained to the south) nor Lake Fazzan in Libya (probably served as an internal basin).
Both the molecular isotope data and satellite data are consistent with a past freshwater corridor out of Africa available for modern humans.
Obtaining further evidence for the migration.
The scientists note that their hypothesis of a freshwater river out of Africa, enabling modern human migration from the continent, is open to further testing. Specifically, scientific analysis of any artifacts along the now buried river could confirm or question their hypothesis. Such testing should be instigated, in order to learn more about modern human origins and migrations.
for more information:
Osborne, A. H.; Vance, D.; Rohling, E. J.;
Barton, N.; Rogerson, M.; Fello, N.
A humid corridor across the Sahara for the migration of
early modern humans out of Africa 120,000 years ago.
Proc. Natl. Acad. Sci. USA 2008, 105,
16444-16447.